Cold-Induced Thermogenesis Shows a Diurnal Variation That Unfolds Differently in Males and Females.

CONTEXT: Cold exposure mobilizes lipids to feed thermogenic processes in organs, including brown adipose tissue (BAT). In rodents, BAT metabolic activity exhibits a diurnal rhythm, which is highest at the start of the wakeful period. OBJECTIVE: We investigated whether cold-induced thermogenesis displays diurnal variation in humans and differs between the sexes. METHODS: This randomized crossover study included 24 young and lean male (n = 12) and female (n = 12) participants who underwent 2.5-hour personalized cooling using water-perfused mattresses in the morning (7:45 am) and evening (7:45 pm), with 1 day in between. We measured energy expenditure (EE) and supraclavicular skin temperature in response to cold exposure. RESULTS: In males, cold-induced EE was higher in the morning than in the evening (+54% ±â€…10% vs +30% ±â€…7%; P = 0.05) but did not differ between morning and evening in females (+37% ±â€…9% vs +30% ±â€…10%; P = 0.42). Only in males, supraclavicular skin temperature upon cold increased more in morning than evening (+0.2 ±â€…0.1 °C vs -0.2 ±â€…0.2 °C; P = 0.05). In males, circulating free fatty acid (FFA) levels were increased after morning cold exposure, but not evening (+90% ±â€…18% vs +9% ±â€…8%; P < 0.001). In females, circulating FFA (+94% ±â€…21% vs +20% ±â€…5%; P = 0.006), but also triglycerides (+42% ±â€…5% vs +29% ±â€…4%, P = 0.01) and cholesterol levels (+17% ±â€…2% vs 11% ±â€…2%; P = 0.05) were more increased after cold exposure in morning than in evening. CONCLUSION: Cold-induced thermogenesis is higher in morning than evening in males; however, lipid metabolism is more modulated in the morning than the evening in females.